1. Field of the Invention
This invention relates to a device for and a method of reproducing a recording medium, and particularly to a device for and a method of reproducing a recording medium, wherein the time required to reproduce an image recorded in the recording medium can be accurately computed and specified.
2. Description of the Related Art
In recent years, a DVD (Digital Versatile Disc) has been standardized and is becoming pervasive. Two types of time information are recorded on this DVD.
One of the two is a time system represented in the form of such a 32-bit integer that one second is expressed by 90000 counts with those obtained by dividing one second into 90000 equal parts as units. This is called "time stamp". Another one is a time system obtained by adding a graph representative of whether one second is defined by 25 frames or 30 frames to the time, minute, second and frame represented by BCD (Binary Coded Decimal) such that one second is expressed by 25 frames or 30 frames with a frame of image as a unit. This is called "time code". This time code can be also expressed as one in which the number of frames has been counted.
The time stamp indicates a relative actual time counted from a given standard time. Thus, no error is produced between the same and the actual time. In contrast to this, the time code is defined as a time system having an error between the same and an actual time because the number of frames is defined as the unit, e.g., the number of frames displayed per second is not an integer accurately in the case of, for example, an NTSC image signal.
FIG. 7 shows a structure of VMG and VTS recorded as a UDF (Universal Disc Format) of a DVD. As shown in the same drawing, the VMG (Video Manager) is recorded at the head thereof and n VTS (Video Title Set) are next recorded behind the VMG.
The head of VMG is defined as a control data area in which Video Manager Information (VMGI) is recorded. The same VMGI is recorded even in a backup area located at the end of VMG. VOBS (Video Object Set) for Menu (VMGM.sub.-- VOBS) is recorded between them.
A control data area is formed at the head of VTS. Video Title Set Information (VTSI) is recorded in the control data area. VOBS for Menu (VTSM.sub.-- VOBS) is recorded subsequent to the control data area. Further, VOBS for Title (VTSTT.sub.-- VOBS) including one to nine files is recorded subsequent to the VOBS for Menu (VTSM.sub.-- VOBS). VTSI identical to that for the control data area is recorded at the end of VTS as backup.
ID numbers for cells included in individual VOB (Video Object) are described in VTSTT.sub.-- VOBS.
PGCI (Program Chain Information) is recorded in VMGI or VTSI in the control data area of VMG or VTS.
FIG. 8 shows an example of a configuration of PGCI. As shown in the same drawing, PGCI comprises a plurality of PGC (Program chains). Numbers of cells constituting the individual PGC are respectively recorded in the respective PGC. Further, time information about PGC Playback Time (PGC.sub.-- PB.sub.-- TM) and Cell Playback Time (C.sub.-- PBTM) are recorded in this PGCI in time code notation. PGC.sub.-- PB.sub.-- TM is one obtained by representing the total playback or reproduction time required to reproduce a series of cells included in the individual program chains in the form of a time code. C.sub.-- PBTM are those obtained by representing times or time intervals required to reproduce the individual cells included in the program chains in the form of time codes.
FIG. 9 shows a hierarchized physical data structure of the DVD. A Video Object Set (VOBS) is composed of a plurality (i in an example shown in FIG. 9) of Video Objects (VOB). Further, each Video Object is comprised of a plurality (j in the example shown in FIG. 9) of cells. Incidentally, VOB.sub.-- IDNk shown in the drawing indicate ID numbers of the video objects respectively and C.sub.-- IDNk indicate ID numbers of the cells respectively.
Each cell is composed of a plurality of video object units (Video Object Units (VOBU)). Further, the video object unit is composed of a navigation pack (Navigation Pack (NV.sub.-- PCK)), audio packs (Audio Packs (A.sub.-- PCK)), video packs (Video Packs (V.sub.-- PCK)) and sub picture packs (Sub Picture Pack (SP.sub.-- PCK)). As shown in FIG. 9, the navigation pack is disposed only at the head of each video object unit.
Audio data, video data and captions data are individually compressed and encoded in the-audio pack, the video pack and the sub picture pack and recorded therein. Added information for smoothly performing processes such as normal reproduction, special reproduction, etc. is recorded in the navigation pack. At least three pieces of time information are recorded in the navigation pack. One of them is Presentation Start Time of VOBU (VOBU.sub.-- S.sub.-- PTM) and another one is Presentation End Time of VOBU (VOBU.sub.-- E.sub.-- PTM). They are those obtained by representing reproduction start times and reproduction end times of their corresponding video object units in time stamp notation. A further one is Cell Elapse Time (C.sub.-- ELTM) which is one obtained by representing a reproduction start time counted from the head of each cell including it, of the corresponding video object unit in the form of a time code.
Meanwhile, the time information represented in the form of the time stamp has no error between the same and an actual time but is entered in the navigation pack alone. Thus, in order to recognize the total reproduction time for the whole cells from the time information expressed in the time stamp rotation, for example, a reference must be made to VOBU.sub.-- S.sub.-- PTM of the navigation pack in the leading video object unit of each cell and VOBU.sub.-- E.sub.-- PTM of the navigation pack in the final VOBU of each cell. In order to recognize the overall reproduction time of each program chain comprising a plurality of cells on a time-stamp basis, the reproduction times must be determined every cells and added up with respect to all the cells included in the program chain. Therefore, a long calculation time interval is required for purposes of obtaining the total reproduction time in the program chain on a time-stamp basis and hence its practicality is next to impossible.
On the other hand, since the time information expressed in time code is recorded in PGCI, a reference may be made to one PGCI to determine the whole reproduction time of program chain. As compared with the case in which the time information expressed in time stamp is obtained, this can be done in an extreme short time.
In the time codes, however, the time is expressed in frame units, whereas image data in each video pack exist in field units. Thus, the time to be recorded inevitably results in time expressed in field units. However, when the time is expressed on a time-code basis, the expression of the time in frame units is not to be avoided. As a result, the time code might have an error of a time interval corresponding to one field.
A method of producing or creating the time information is not defined in the format of the DVD. Therefore, time errors each corresponding to one field are cumulated depending on the creating method in the case of the time information expressed in the time codes. Thus, a large error might be produced between the number of frames expressed in time codes and the number of frames displayed in practice.
Now consider where as shown in FIG. 10 by way of example, the number of fields of an image, which is contained in one video object unit, is 61 fields (1 second +1 field) and reproduction times in individual video object units are represented in time codes. Assuming that the remaining one filed is cut off, an error of one field is produced with respect to one video object unit. Assuming that 60 video object units exist in one cell, the total reproduction time expressed in the time code, per cell (corresponding to 60 video object units) has an error of 60 fields at the maximum.
Thus, if, for example, time codes are created every video object units and C.sub.-- ELTM or C.sub.-- PBTM is produced from the sum of them, then a large error is produced between C.sub.-- ELTM or C.sub.-- PBTM and the actual reproduction time.
Similarly, the error exists even between each cell and the program chain. Namely, now consider that as shown in FIG. 11, the number of fields of images included in individual cells is 3601 fields (1 minute+1 field) and the reproduction time for each cell is expressed in form of a time code. Assuming that the remaining one field is omitted or cut off, the reproduction time obtained in the time code becomes shorter than a reproduction time acquired from the displayed number of frames. Assuming that one program chain is comprised of 100 cells as shown in FIG. 11, its error results in 100 fields at the maximum.
Thus, if the time codes are created every cell and PGc.sub.-- PB.sub.-- TM is produced from the sum of them, then a large error is produced between the value thereof and a reproduction time obtained from the displayed number of frames of an image.
Assuming that as shown in FIG. 12, for example, one cell is composed of 60 video object units, each video object unit is composed of 61 fields and each time code is calculated by cutting off the remaining one field, the time required to actually display one cell results in 1 minute and 1 second (3660 fields) but the reproduction time expressed in the time code reaches 1 minute. As a result, an error of 60 fields is produced per cell. Thus, assuming that 60 cells are reproduced, an error of 3600 fields is produced.
When the reproduction time is calculated using the time information expressed in the time code notation as described above, a plurality of reproduction times are obtained for each program according to the difference between the computing methods. Thus, when the present reproduction time (corresponding to a reproduction time from the head of a program to the present display position) of a predetermined DVD is displayed on a reproducing device reproducing the DVD, for example, the displayed reproduction time differs according to the difference between the reproduction time computing methods even if the same program is displayed.
Thus, when a reproduction start position is specified by the reproduction time as counted from the head of the program, for example, the position to actually start the reproduction differs according to the reproducing device even if the same time is specified. Consequently, when a position set proximately to the completion of the program is specified as the reproduction start position at worst, a situation in which an image corresponding to a specified time thereof does not exist, is also brought about.
Typical ones of various inventions of the present application have been shown in brief. However, the various inventions of the present application and specific configurations of these inventions will be understood from the following description.